The invention relates to a process for recovering rare metals from the combustion residues of various coals, in particular brown coals. The term "combustion residue" as used throughout the specification includes fly ash, ash, slag and the like.
For a long time coal was almost exclusively utilized as a source of energy. Later it was discovered that coal contains numerous valuable materials, particularly metals of columns Vb and VIb of the periodic system of elements as well as various lanthanides and actinides. Hereinafter these metals will be referred to as "rare metals".
These rare metals are enriched in the combustion residue of coal to a concentration which is more than 200 to 500-times higher than their average concentration in the earth's crust. For example the combustion residue of brown coal from the Eocene epoch in Hungary contains more than 2000 to 3000 ppm of vanadium, 2000 to 5000 ppm of chromium, 100 to 200 ppm of molybdenum, more than 2000 ppm of lantanides and more than 1200 ppm of actinides.
There are processes known in the art for the isolation of rare metals from coals, they are, however, extremely expensive. In certain cases the treatment has an influence on the use of the coal as a source of energy. In other cases the combustion residues need to treated with various further additives to isolate the rare metal, and accordingly, the energy demand of the whole process is considerably increased.
A process is disclosed in Soviet Pat. No. 256,261 for recovering vanadium. According to this patent the coal is admixed with calcium oxide or calcium carbonate prior to combustion, or these additives are added to the ash. The mixture of ash and the additive is then heated to a temperature of 850.degree. C. and then it is treated with a 3% aqueous sodium carbonate solution, at a temperature of 98.degree. C. As a result of this treatment vanadium is dissolved in the form of NaVO.sub.3. The residue is then treated with a 5% aqueous sulfuric acid solution to dissolve any undissolved vanadium remaining therein.
According to a process disclosed in U.S. Pat. No. 4,130,627, coal ash is heated up to 90.degree. C. to 135.degree. C. whereupon it is treated with alkaline reactants. An alkaline aqueous mixture and a solid residue are separated by filtration.
Iron and aluminium are dissolved from the solid residue with hydrochloric acid or hydrofluoric acid as the corresponding chloro and fluoro compounds. Alumina is obtained by electrolysis and silicon remains at the bottom of the electrolyzer. The main object of this process is the recovery of the main components of ash, i.e. the production of alumina and high-grade silica powder. Though the possibility of isolating other metals is also alluded to, there are no specific hints how to proceed when the recovery of further components is not desired. A characteristic feature of the process is that the combustion residue is completely (or almost completely) digested and accordingly, all components of the combustion residue are dissolved. The separation of the valuable rare metals from accompanying substances requires further, rather complicated technological steps, which are not disclosed in the cited patent and have an extremely high energy demand.
Although the more valuable components can be recovered more selectively by chlorination of the ash and fly ash, due to the high reactivity of the reactants used, the necessary apparatus is very expensive. Moreover, by these processes only certain rare metals can be isolated, they are not suitable for a complex recovery of rare metals.